The invention relates to a soft start device for pneumatic systems and to a method for the operation of a soft start device. The soft start device comprises:                a primary inlet where compressed air can be supplied at primary pressure,        wherein the primary inlet is, via a valve circuit, connected to at least one secondary outlet which can be coupled to a load and where compressed air can be discharged at secondary pressure, the secondary pressure being lower than or equal to the primary pressure,        wherein a main valve of the type 2/2 nc (normally closed) which can be bypassed by means of a bypass is installed between the primary inlet and the secondary outlet, a restrictor device being installed in the bypass,        wherein the main valve and the restrictor device together with further directional valves of the valve circuit are interconnected such that, in a soft start process while the main valve is initially closed, compressed air is applied to the secondary outlet at a secondary pressure which is lower than primary pressure and increases gradually, until from a defined ratio between secondary and primary pressure the main valve is switched into its open position, so that compressed air arrives at the secondary outlet at primary pressure, and so that        the valve circuit can be placed in a standard switching position for venting, so that the secondary outlet is vented.        
Soft start devices are used in pneumatic systems to supply compressed air to functional units sensitive to pressure surges, such as service units etc., in conditions where the pressure gradually increases from a relatively low secondary pressure to the higher primary or operating pressure. In this way, pressure surges at harmfully high primary pressure are avoided. Functional units sensitive to pressure surges include filter units or double-acting pneumatic cylinders. In the case of double-acting pneumatic cylinders, the piston may be in an intermediate position in the “pressure-less” state of the cylinder, so that the piston, if subjected to the full pressure surge, could suddenly move into one of its end position, which may result in damage to the piston or to the end stop of the cylinder. The unintended piston movement could moreover damage a downstream functional unit, which may once again result in a hazardous situation. Such dangerous movement could result in personal injury in particular. This is prevented by a soft start, causing the piston to move into its end position relatively slowly.
A soft start device is for example disclosed in EP 0 758 063 B1, which describes a starting valve in the form of a seating valve, the valve being vented via a fast venting device. The starting valve comprises a housing wherein a single flow path runs from the inlet to the outlet, the seating valve acting as a restrictor being disposed in the flow path.
In pneumatic systems, certain safety aspects have to be observed. These are for example categorised in the standard EN 954-1 and in the follow-up standard DIN EN ISO 13849-1. To fulfil the requirements of category 3 of EN 954-1, the pneumatic device is subject to so-called “single fault safety” in safety-relevant functions. This means that the system must be capable of venting even if a single fault is present.
A soft start device of the type referred to above which offers “single fault safety” is disclosed in EP 645 755 A2. If one of the valves malfunctions, the secondary outlet can nevertheless be vented. This being so, this soft start device fulfils the requirements of EN 954-1, category 3.
The follow-up standard DIN ISO 13849, however, requires a higher test quality for category 3 and in particular for category 4. It demands in particular that a test or diagnostic mode of the valves should be run prior to the soft start process, in order to eliminate faults in the initiation of the soft start process.